The Internet includes a system of networks connected together. At least some of the networks may be incompatible with other networks. For example, some networks may operate using IPv6 (described in RFC 1883 (1995)) and other networks may operate using IPv4 (described in RFC 791 (1981)). IPv6 is designed to succeed IPv4, which is the dominant protocol version currently in use by clients. Since the number of addresses in the IPv4 address space is limited and is reaching exhaustion, a gradual move to IPv6 is occurring as one of the main advantages of IPv6 is that it supports many more addresses than IPv4. IPv6, however, does not directly interoperate with IPv4. For example, clients that support only IPv4 cannot directly communicate with IPv6-only servers, and IPv6-only clients cannot directly communicate with IPv4-only servers. In the Domain Name System (DNS), the “A” resource records map hostnames (domain names) to IPv4 addresses and the “AAAA” (quad A) resource records map hostnames to IPv6 addresses.